1. Field of the Invention
The present invention is concerned with a steam generator comprising an enclosure containing a liquid to be evaporated and, within this enclosure, a module for rapid evaporation comprising at least one heating electrical resistance housed in a porous body immersed in the liquid to be evaporated or located near this porous body to heat the same by radiation.
2. Description of the Prior Art
Generally, most steam generators operating in industrial and household domains are used only on an average of 20 to 50% of their normal outputs. Such for example is the case with industrial or household humidifiers. The units used in the latter application comprise generally an enclosure containing liquid to be evaporated, which is usually water, maintained at a constant level by a level regulator in such a way that the resistance is constantly immersed in the water to be evaporated. Generally, these devices comprise systems which automatically and periodically control the cleaning cycle of these enclosures in which mud accumulates and also cycles for eliminating lime which adheres to the surface of the body of the resistances and which is broken by overheating in the absence of water. The production of the steam is monitored by a hydrometric probe which controls an on and off basis for the operation of the boiler, through a device which ensures passage of electric current in the resistance.
Such devices have the primary drawbacks of requiring a steam start up time which is relatively long when starting from the cold state or after these cycles of cleaning and of calcium elimination and of having a response time which does not permit proper regulation of the steam output while also producing a steam which contains suspended droplets of water; requiring a sophisticated control system and resistances of the highest quality when the sheet of lime is destroyed. Moreover, should such devices be equipped with energy metering devices, the transfer times then would be considerably increased by the presence of a large volume of water when operating with small or average power in the heating resistance. These devices are relatively expensive to manufacture and to operate while producing a mediocre quality of steam owing to its moisture content, with response times which are very lengthy.
Additionally, it is known that lime accumulation and the hardness of the lime deposited onto the surface of the resistance shorten its life and causes delays in thermal transfer times and a loss in energy production. Contrariwise, by combining porous bodies with the surface of the resistance, it is noted that lime no longer is deposited on one surface, as is the case with the surface of the resistance when the latter is used alone, but in the three-dimensional volume constituting the porous body and that this lime has a spongy consistency which is no longer very hard for the same duration of operation, which therefore lessens the drawbacks previously mentioned. Also known is the absorption power of the porous bodies, their pumping and capillary capacities as well as the volume distribution of the liquids in proportion to the height of the pumping. It is also known that by combining porous bodies with heating sources the exchange coefficients are thereby considerably increased and there is obtained a relatively short response time. As is described in French Patents FR-A-2,341,330 and FR-A-2,420,731 it has already been proposed to use porous bodies that are made to function in steam generators by immersing a porous body, equipped with an internal electrical resistance in a permanent water layer.